Candida species are able to colonize and infect many sites throughout the human body. The most clinically relevant species is Candida albicans, which is a natural commensal of humans and also the most frequently encountered fungal pathogen. C. albicans is the primary cause of both mucosal and systemic fungal infections, with the latter being associated with high rates of mortality. The prevalence of C. albicans in the clinic is a consequence of its being able to adapt to different niches in the host, as well as to environmental stresses such as antifungal drugs. Most pathogenic fungi were originally designated as asexual species, yet it is now recognized that many have retained the ability to generate diversity by undergoing a sexual or parasexual program. In C. albicans, an unusual parasexual mating cycle has been defined, which is regulated by a phenotypic switch between white and opaque states. Mating occurs by conjugation between diploid opaque cells to generate tetraploid mating products. Tetraploid cells can then reduce their ploidy by the process of concerted chromosome loss (CCL) to generate diploid and aneuploid progeny. Our studies will address each of the steps in the parasexual lifecycle, including the role of the host in facilitating this process, as well as defining how the aneuploid products of parasex can impact disease. The Specific Aims are: (1) To determine the mechanism by which the white-opaque switch regulates mating in C. albicans. We will test the hypothesis that reduced MAPK signaling plays a key role in limiting mating by white cells, thereby promoting a predominantly clonal lifestyle. (2) To define the niches in the mammalian host supporting the parasexual cycle. While detailed studies of mating have been performed in vitro, the in vivo conditions favoring parasex remain unclear. An analysis of parasex will be performed in different murine infection models, and will build on observations that parasex occurs efficiently in a commensal niche. (3) To utilize parasexual genetics to study the role of aneuploidy in C. albicans. Aneuploid forms of C. albicans exhibit clinically relevant phenotypes including increased antifungal drug resistance. We will analyze parasexual progeny carrying different supernumerary chromosomes to address the phenotypic properties associated with aneuploidy, including their impact on pathogenesis. Throughout the proposal cutting-edge techniques such as ddRad-Seq and TrackScar will be emphasized as innovative tools to study the biology of pathogenic yeast. The proposed studies will provide new mechanistic insights into the unique parasexual cycle of C. albicans and the diversity of progeny generated by this program in the mammalian host. Experiments will also utilize parasexual genetics as a tool for addressing the role(s) of aneuploidy in C. albicans, including their potential to act as important intermediates in the evolution of antifungal drug resistance.